Impact of Co3O4 nanoparticles on epoxy's mechanical and corrosion-resistance properties for carbon steel in seawater

Co3O4 nanoparticles (Co3O4-NPs) are synthesized using the facile solvothermal method. FT-IR and XRD spectroscopic analyses verify the creation of cobalt oxide nanoparticles with an average size of 13.20 nm. Furthermore, Zeta potential assessments were carried out to identify the electrical charge of the surface of the produced Co3O4-NPs, which was found to be -20.5 mV. In addition, the average pore size of Co3O4-NPs is 19.8 nm, and their BET surface area is 92.4 m/g. The study also concerned the effect of Co3O4-NPs on epoxy's improvement of mechanical and corrosion protection for carbon steel in salt solution. By including Co3O4-NPs in an epoxy (EP) coating, corrosion is effectively prevented by non-permeable protective coatings that effectively reduce the transfer of corrosion ions and oxygen.


Synthesis of Co 3 O 4 -NPs
The solvothermal method 21 was implemented for the preparation of cobalt oxide nanoparticles (Co 3 O 4 -NPs) by dissolving (0.024 mol, 0.678 g) of Co(NO 3 ) 2 .6H 2 O in 12 ml of ethyl alcohol forming red color solution (A).In another 250 ml conical flask, prepare 0.09 mol of sodium hydroxide by dissolving appropriately in ethanol, forming a colorless solution (B).Then, solution (A) was added drop by drop to solution (B) during vigorous stirring using a magnetic stirrer.The previous mixture was allowed to complete for 20 min by forming blue color solution (C).The reaction was then completed by transferring mixture (C) after adding 0.6 g CTAB (prevent aggregation of the NPs) to 100 ml autoclave with Teflon linear and heated for five hours at 180 °C.Finally, black powder of Co 3 O 4 nanoparticles was obtained after washing several times using ethanol and drying in the oven.

Preparation of Co 3 O 4 -NPs@EP coating
The Co 3 O 4 -NPs@EP coating was created by combining EP resin from Ciba, poly-amidoamine (hardener) from Arkema, and 2.5 wt.% Co 3 O 4 -NPs.When a high concentration of Co 3 O 4 -NPs is added (more than 2.5 wt.% Co 3 O 4 -NPs), agglomerates form, causing the dispersion to be deemed insufficient to be used any further quantity.
The EP resin to hardener weight ratio became 2:1.All of the components were blended for 3 h with a speed mixer (1300 rpm) 22,23 .High-purity N 2 flow was bubbled into the mixture while stirring.
The uniform dispersion of Co 3 O 4 -NPs in EP resin was checked using a scanning electron microscope (ZEISS scan electron microscopy, SEM). Figure 1a illustrates that the surface of the neat EP resin is smooth and free of impurities.In contrast, spherical particles are visible on the Co 3 O-NPs@EP surface (Fig. 1b).In EP resin, the Co 3 O 4 -NPs particles were dispersed uniformly.
Before coating, the metal sheets were degreased with acetone and ultrasonic cleaning with 95% ethanol, then with de-ionized water and drying.
The clean substrate was covered with Co 3 O 4 -NPs@EP coatings using a film applicator 24 .The dry film thickness of EP resin and Co 3 O 4 -NPs@EP coating was determined using a coating micrometer of 45.2 ± 3 and 63 ± 4 μm, respectively.

Corrosion and mechanical tests
As the working electrode, a carbon steel panel was employed to evaluate the resin's resistance to corrosion in 3.5% NaCl solution.
The EIS was applied using a Gamry-Interface-5000E potentiostat/galvanostat to investigate the coating system's effectiveness.For EIS experimental tests, an electrochemical cell defining a set was used.The platinum (counter electrode), coated carbon steel (working electrode), and saturated calomel (reference electrode) electrodes were used to build the cell.
The exposed surface of the coating is one side with a dimension of 30 mm × 10 mm during the EIS measurement.Impedance spectra were recorded at open circuit potential (6 h) with frequencies ranging from 1 Hz to 30 kHz and potential amplitudes equal to 10 mV.After seven days of immersion, the test was carried out.The nano-indentation method was used to evaluate mechanical characteristics (Micro Materials instruments).Using Micro Materials instruments, the nano-indentation test was carried out on the nanocomposite sample in three stages: loading, holding, and unloading 25 .The international standard ISO 14577 governs these experiments.Impact resistance and scratch-hardness tests were performed by ASTM specifications (ASTM-D2794, ASTM-D7027) 26 .The experiments were done in triplicate, and the outcomes were very reproducible.
The salt spray experiment (3.5 wt.% solution) was performed in a corrosion tester cabinet at 323 K in accordance with ASTM B117.After 168 h, the samples surfaces and degree of rusting were inspected visually and evaluated.

Methods of characterization
FT-IR spectroscopic measurement was obtained using a Perkin Elmer-Spectrum spectrophotometer on the prepared Co 3 O 4 -NPs (KBr pellet technique).Spectral data were gathered from 400 to 4000 cm −1 .Functional reference spectra were used to determine groups.The produced nao-metal oxide' crystallinity was proven using an X-ray diffractometer (XRD).The applied wavelength (λ) was 1.5418 Å using Cu Kα radiation (PANalytical XPERT PRO MPD, Netherlands).At room temperature, the diffraction pattern is recorded in the angular range (2θ) of 10-80 with a step size of 0.02.In addition, zeta potential measurements (Malvern Zetasizer ZS-HT, UK) were used to identify the anti-aggregation resistance of the synthesized metal oxide nanoparticles.The measures depend on the electrophoresis where the value of zeta potential is related to the electrophoresis (Henry equation) 27 .
The N 2 gas adsorption/desorption performance and specific surface area of the resulting material were studied using Brunauer-Emmett-Teller (BET) analysis with a Quantachrome NOVA Station A.
where, λ is the wavelength of the X-ray radiation source (0.15405 nm), θ is the half diffraction angle (also known as the Bragg angle), and β is the full width at half-maximum value (FWHM) in radians of the XRD diffraction lines.The previous equation declares that the peak width, as measured perpendicular to the reflecting planes, is inversely related to crystal thickness 32 .The mean diameter of the synthesized Co 3 O 4 -NPs is calculated and found to be 13.20 nm depending on (311).The recorded FT-IR spectrum (Fig. 3) is used to obtain structural data from the functional groups of the produced Co 3 O 4 -NPs.Strong metal oxide bands at 666.63 cm −1 and 574.03 cm −1 are related to Co(III) in an octahedral and Co(II) in a tetrahedral site 33,34 .In addition, the peak present at 1390 cm −1 regards to C-H bending of CTAB that capped the formed nanoparticles 35 .
Zeta potential is a physical property defined as the electrical potential between the investigated material and the surrounding liquid 36 .Figure 4 shows the zeta potential distribution with a mean value of −20.5 mV.Zeta potential is a good tool that can determine the particles' electrical surface charge 37 .When an electric field is applied over the scattered Co 3 O 4 -NPs, the nanoparticles will move toward the oppositely charged electrode at a rate proportional to the zeta potential.High zeta potential for a dispersion system, whether positive or negative, suggests that the particles are resistant to aggregation and indicates an apparently stable system 38 .The surface of synthesized Co 3 O 4 -NPs is the negative value of zeta potential which agrees with the literature 39,40 .
Co 3 O 4 -NPs have a BET surface area of 92.4 m 2 /g and an average pore size of 19.8 nm.

Corrosion protection features of Co 3 O 4 -NPs@EP coating
This section used EIS plots to evaluate the effectiveness of EP-coated carbon steel in both the incorporation and absence of Co 3 O 4 -NPs in 3.5% NaCl solution (see Fig. 5).As depicted in Fig. 5, two capacitive loop circuits make up the Nyquist, Bode-module, and phase angle plots for both coatings (Fig. 5a-c).The two capacitive circuits at high and low frequencies are induced by the capacitance and resistivity of the EP covering and the steel/electrolyte interface, respectively 41,42 .Figure 6 depicts an equivalent electric circuit that utilizes two-time constants for both coatings.Electrolyte resistance (R s ), charge-transfer resistance (R ct ), coating resistance (R c ), coating capacitance (C c ), and double-layer capacitance (C dl ) are all present in this structure 43 .The coating capacitive circuits' radius was extended by adding 2.5 wt.% Co 3 O 4 -NPs (Fig. 5).The R c and C c qualities for neat EP coating are 15.3 MΩ cm 2 and 1.6 × 10 -8 F cm −2 , respectively.In addition to an increase in R c to 84.4 MΩ cm 2 and a decrease in C c value to 0.78 × 10 -9 F cm −2 , in the Co 3 O 4 -NPs@EP coating.Co 3 O 4 -NPs coating seemed to have a greater phase angle than a neat EP coating, denoting that it could be quite resistant (see Fig. 5c).In the case of neat EP coating, it is nearly impossible to prevent significant moisture transport through coating layers to control the corrosion process 10 .The electrochemical processes that take place at the defect points in the coating layer can be outlined using the equations below 44,45 : (2) Fe (s) → Fe 2+ + 2e at anodic sites  www.nature.com/scientificreports/When Co 3 O 4 -NPs are added to an EP coating, impermeable protective coatings are created that effectively block the transfer of corrosion ions and oxygen, preventing corrosion from occurring.
Co 3 O 4 -NPs can act as barriers within the epoxy matrix, hindering the movement of corrosive species such as water, oxygen, and ions.This physical barrier reduces the contact between the metal substrate and the corrosive environment, thereby slowing down the corrosion process 46 .
Using the nano-indentation method, the mechanical characteristics of carbon steel with EP coating in both the absence and addition of Co 3 O 4 -NPs were examined.Loading-unloading charts for carbon steel surfaces coated with neat EP and Co 3 O 4 -NPs@EP are seen in the Fig. 7.
Evidently, the EP coating adapted with Co 3 O 4 -NPs provided markedly improved EP coating hardness (from 126 to 445 mN/m 2 ).This trend could be attributed to the Co 3 O 4 -NPs' propensity to fill in cracks and open spaces in the EP coating, which lowers the overall free volume and raises the cross-linking density of the dried epoxy 47,48 .The physicomechanical features of carbon steel surfaces coated with neat EP and Co 3 O 4 -NPs@EP are described in Table 1.
It's worth noting that incorporating Co 3 O 4 -NPs into the neat EP positively affects the EP coating's scratch resistance.The scratch hardness of Co 3 O 4 -NPs@EP was the highest.The growth in scratch hardness could be associated with a decline in indentation caused by rising physical interaction between the EP resin and the Co 3 O 4 -NPs.Co 3 O 4 -NPs appear to be included in the EP polymer backbone to restrict chain mobility, leading to high-impact resistance (see Table 1).The dispersion of Co 3 O 4 -NPs within EP resin and the powerful interactions of Co 3 O 4 -NPs and EP resin with the epoxy polymer are the main considerations for improving the mechanical features of Co 3 O 4 -NPs@EP coating 49,50 .
The salt spray experiment was carried out for 168 h to assess the impact of Co 3 O 4 -NPs introduction on the corrosion inhibition behavior of the EP resin coating.The various observable corrosions that formed across the scratch, as seen in Fig. 8a, indicate that neat EP seems to have poor corrosion inhibition features.The addition of Co 3 O 4 -NPs to the EP coating significantly minimized the degree of corrosion throughout the scratch of the coating (Fig. 8b).As a result, adding Co 3 O 4 -NPs will improve the compactness and the corrosion inhibition of EP coating.

Conclusion
Co 3 O 4 -NPs were prepared by solvothermal method and then analyzed by FT-IR and XRD spectroscopic measurements.In a brine solution, the effect of Co 3 O 4 -NPs on the improvement of epoxy's corrosion protection and mechanical performance for carbon steel was investigated.For a neat EP coating, the R c and C c qualities are 15.3 MΩ cm 2 and 1.6 × 10 -8 F cm −2 , respectively.Two capacitive circuits can be seen in the Co 3 O 4 -NPs@EP coating, with R c increasing to 84.4 MΩ cm 2 and C c decreasing to 0.78 × 10 -9 F cm −2 .It was clear that the EP coating that had been modified with Co 3 O 4 -NPs increased the hardness of the EP coating (from 0.126 to 0.445 GPa).Incorporating Co 3 O 4 -NPs into an epoxy (EP) coating produces non-permeable protective layer that effectively inhibits the transmission of corrosion ions and oxygen, stopping corrosion.

Figure 5 .
Figure 5. EIS spectra (a) Nyquist, (b) Bode-module, (c) Bode-phase angle plots for carbon steel coated with neat EP and Co 3 O 4 -NPs@EP coating in 3.5% NaCl liquid at 298 K after 7 days of immersion.

Figure 6 .
Figure 6.Equivalent circuit for fitting of the impedance data.

Figure 7 .
Figure 7. Loading-unloading charts for carbon steel surfaces coated with neat EP and Co 3 O 4 -NPs@EP.

Figure 8 .
Figure 8. Photographs of cyclic salt-spray test in the case of (a) EP resin and (b) Co 3 O 4 -NPs@EP.

Table 1 .
Physicomechanical features of carbon steel surfaces coated with neat EP and Co 3 O 4 -NPs@EP.